The present invention is related to an apparatus and a method for the measurement of the vertical position of upper and lower punches, pairwisely associated to a rotatably driven rotor of a rotary tablet press and rotating synchronously with the rotor, which perform a vertical movement in certain regions along the circumference of their rotational movement during a rotation of the rotor. Rotary tablet presses are known since a long time. The rotor of such presses has usually a die plate with a plurality of dies spaced apart over its circumference. In this, the rotor is equipped with a number of upper and lower punches forming compression punches of the press, wherein one lower punch and one upper punch is associated to each die. During one rotation of the rotor, each punch is subjected to a vertical movement at certain positions of the circumference, at the compression stations of the press in particular, which leads to a corresponding action of force to a material to be compressed which is filled into the dies. Besides to a precompression station and a main compression station, an ejection station must also be taken into consideration. The vertical movement of the compression punches is usually guided by cam curves. By for instance stationary pressing rollers, a pressing force is applied to the heads of the compression punches in the compression stations, and through this to the material which is to be compressed. Depending on the composition of the tablets to be pressed, a preset pressing force has to be maintained in this process.
Pressing force measurement devices are used in rotary tablet presses since many years for monitoring and controlling the compression process, in order to ensure the desired tablet quality. However, in the field of pharmaceutical laboratory and development work, it is not always sufficient to judge the compression process via the measurement of the pressing force. Other measurement variables have therefore to be used. The force-path diagram has an eminent importance for the determination of the compressions characteristics of pressed materials. Seen from the physical point of view, it describes the energy expenditure during the compression phase. From this, conclusions regarding the pressability and characteristics of the substances to be compressed can be drawn.
Different devices for the measurement and the detection of the punch path have become known. For instance, in DE 19502596 C2, the entire contents is incorporated herein by reference, it is proposed to calculate the punch path theoretically, based on the geometrical conditions of the press. A metrological approach is described in DE 102005051567 A1, the entire contents is incorporated herein by reference. In this, the distance to a marker in the form of a horizontal plate fixed on the punch is measured during the compression phase with a preferably stationarily arranged and contactless working sensor. Via the distance measurement, the compression phase of the punches can be monitored, i.e. the vertical movement in the compression stations. However, desired or undesired movements of the compression punches can occur beyond the compression phase, the measurement of which may also be required.
Devices in which inductive or ohmic path sensors are mounted before or behind the punch which is to be measured have also become known. However, these devices are sumptuous in the practical use.